Of course, fish is an animal. It belongs to the subphylum Vertebrata of the phylum Chordata. It is the oldest vertebrate. This kind of animal is found in almost all aquatic environments on earth, some in the sea and some in fresh water. At present, they are mainly divided into two categories, one is cartilaginous fish, the other is bony fish.
1. Breathing with gills: Both bony fish and cartilaginous fish have a characteristic, that is, breathing with gills. Gills usually grow on both sides of the head, water enters from the mouth, then flows out through the two gills, and when flowing out, the gill filaments in the gills can complete gas exchange, thus obtaining oxygen and discharging carbon dioxide at the same time.
2. Scales on the body surface: Not all fish have scales, but scales are a major feature of fish, and they are basically scaleless fish. Some are not completely scaleless, but have fewer scales. Fish scale is its skin derivative, itself is calcium, covering the whole body or part of the fish, can play a role in protecting the body.
3, swim by fins: Fish live in the water, swimming is not on the limbs, but the fins. There are many kinds of fins, namely pectoral fin, pelvic fin, anal fin, dorsal fin and caudal fin, of which the caudal fin can provide forward power for the fish, but also control the direction of swimming, the other fins are mainly to help maintain balance.
4. Oviparous reproduction: Most fish are oviparous, and a few are ovoviviparous. Although it looks like viviparity, in fact, the fish eggs only develop in the body, which is different from the real viviparity. In addition, there are a very small number of bony fish that develop by metamorphosis.
